.alpha.-L-aspartyl-L-phenylalanine methyl ester (hereinafter abbreviated as .alpha.-APM) is a compound commonly referred to as "aspartame" and is an important substance as a sweetening agent. It has a sweetness approximately 200 times that of cane sugar and thus is in ever-increasing demand as a diet sweetening agent.
Numerous chemical preparation processes of .alpha.-APM have so far been disclosed. There may be cited a variety of processes, for example, (1) a process which comprises condensating the hydrochloride of aspartic acid anhydride and L-phenylalanine methyl ester (see, for example, Japanese Patent Publication No. 40069/1976), (2) a process which comprises condensating N-protective aspartic acid anhydride and L-phenylalanine methyl ester followed by de-protection (see, for example, Japanese Patent Laid-Open Nos. 1370/1971 and 113841/1976), (3) a process which comprises reacting N-protective aspartic acid-.beta.- benzyl ester with L-phenylalanine methyl ester in the presence of a condensating agent followed by de-protection (see Japanese Patent Laid-Open No. 130846/1984) and (4) a process which comprises reacting N-carboxyaspartic acid anhydride and L-phenylalanine methyl ester (see Japanese Patent Laid-Open No. 96557/1973).
However, all of these processes employ L-phenyl-alanine methyl ester as one of the raw materials, requiring complex steps of methyl-esterification of L-phenylalanine prior to the reaction with the active derivatives of aspartic acid.
Moreover, further investigation has revealed that L-phenylalanine methyl ester is a compound in which its two free molecules in a solution are liable to condensate and cyclize to 2,5-dibenzyl-diketopiperazine and thus involves disadvantages in its stability in a solution. This fact may be responsible for various disadvantages, for example, those in the quality of .alpha.-APM, taking place in its industrial production.
Accordingly, it is desirable to develop a process which is free of these disadvantages in the production of .alpha.-APM, i.e., a process in which L-phenylalanine, not L-phenylalanine methyl ester, is used as a raw material.
As a process for preparing .alpha.-APM which does not employ L-phenylalanine methyl ester, there has been disclosed a process for preparing .alpha.-APM which comprises condensating N-formyl aspartic acid anhydride with L-phenylalanine in glacial acetic acid to form N-formyl-.alpha.-L-aspartyl-L-phenylalanine, deformylating the N-formyl-.alpha.-L-aspartyl-L-phenylalanine to form .alpha.-L-aspartyl-L-phenylalanine, and esterifying the .alpha.-L-aspartyl-L-phenylalanine to form .alpha.-APM (see Japanese Patent Publication No. 26133/1980), followed by a process in which the esterification step of the foregoing process is improved (see Japanese Patent Laid-Open No. 82752/1978).
However, since the former process brings about the esterification reaction in a substantially non-aqueous solution, the reaction has practically no freedom of selectivity so that not only the intended esterification of the .alpha.-carboxyl group of aspartic acid but also the esterification of its .beta.-carboxyl group as well as the diesterification of its .alpha.- and .beta.- carboxyl groups takes place to a large extent. Therefore, the process suffers such disadvantages as low selectivity and low yield of .alpha.-APM.
Further, owing to the complexity of isolating .alpha.-APM from the reaction solution after completion of the reaction, the former process is not an industrially successful process.
Given the fact that the hydrochloride of .alpha.-APM is relatively insoluble in aqueous hydrochloric acid solution, the latter process brings about the esterification of .alpha.-L-aspartyl-L-phenylalanine in the co-presence of water to deposit .alpha.-APM hydrochloride out of the reaction system. However, the yield of isolation of .alpha.-APM is at most 50-60% (based on .alpha.-L-aspartyl-L-phenylalanine) and thus is insufficient.
Further, when the reactions are effected in the same reactor in such a way that .alpha.-L-aspartyl-L-phenylalanine is first formed therein from N-formyl-.alpha.-L-aspartyl-L-phenylalanine and then esterified to produce .alpha.-APM, the deformylation reaction of N-formyl-.alpha.-L-aspartyl-L-phenylalanine must be practiced in the presence of a small amount and a low concentration of hydrochloric acid in order to suppress the cleavage of the peptide linkage.
Moreover, the subsequent esterification reaction must be carried out in a system containing small amounts of methanol, water and hydrochloric acid because the whole system has to be compact.
In this case, N-formyl-.alpha.-L-aspartyl-L-phenylalanine used as a raw material is relatively insoluble in the aqueous hydrochloric acid solution containing a small amount of methanol. Thus, such a small amount of the solvent causes the reaction mixture to form a slurry after the charge of the raw material. Therefore, various problems are liable to occur in the course of industrial operation such as in stirring.
Further, the latter process discloses that the reaction conditions have to be selected so as to suppress the formation of unfavorable .alpha.-L-aspartyl-L-phenylalanine dimethyl ester as much as possible.
Thus, the conventional processes for producing .alpha.-APM have merits and demerits in the stability of its intermediate or in reaction operation or in yield and thus are not always satisfactory.
Further, a new preparation process of .alpha.-APM in which .alpha.-L-aspartyl-L-phenylalanine dimethyl ester is hydrolyzed in an aqueous methanol-hydrochloric acid solution has lately been disclosed (see Japanese Patent laid-Open No. 219258/1984). According to this disclosure, the starting .alpha.-L-aspartyl-L-phenylalanine dimethyl ester is prepared by de-formylation and esterification of N-formyl-.alpha.-L-aspartyl L-phenylalanine methyl ester or by condensating the N-carboxylic acid anhydride of L-aspartic acid-.beta.- methyl ester with L-phenylalanine methyl ester. Thus, the process employs L-phenylalanine methyl ester for the preparation of the starting .alpha.-L-aspartyl-L-phenylalanine dimethyl ether and therefore involves the above-described problems.